Applicant's invention relates generally to radiocommunication systems and more particularly to radiocommunication systems in which evaluations of remote unit servers for potential control channel reselection are performed.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One way to increase capacity is to use digital communication and multiple access techniques such as TDMA, in which several users are assigned respective time slots on a single radio carrier frequency.
In North America, these features are currently provided by a digital cellular radiotelephone system called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard IS-54B, "Dual-Mode Mobile Station-Base Station Compatibility Standard", published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), IS-54B is a dual-mode (analog and digital) standard, providing for analog compatibility in tandem with digital communication capability. For example, the IS-54B standard provides for both FDMA analog voice channels (AVC) and TDMA digital traffic channels (DTC), and the system operator can dynamically replace one type with the other to accommodate fluctuating traffic patterns among analog and digital users. The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (KHz).
In a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user.
The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel, which may be considered a logical channel assigned to the user. As described in more detail below, digital control channels (DCCs) can also be provided for communicating control signals, and such a DCC is a logical channel formed by a succession of usually non-consecutive time slots on the radio carrier.
According to IS-54B, each TDMA frame consists of six consecutive time slots and has a duration of 40 milliseconds (msec). Thus, each radio channel can carry from three to six DTCs (e.g., three to six telephone conversations), depending on the source rates of the speech coder/decoders (codecs) used to digitally encode the conversations. Such speech codecs can operate at either full-rate or half-rate, with full-rate codecs being expected to be used until half-rate codecs that produce acceptable speech quality are developed. A full-rate DTC requires twice as many time slots in a given time period as a half-rate DTC, and in IS-54B, each radio channel can carry up to three full-rate DTCs or up to six half-rate DTCs. Each full-rate DTC uses two slots of each TDMA frame, i.e., the first and fourth, second and fifth, or third and sixth of a TDMA frame's six slots. Each half-rate DTC uses one time slot of each TDMA frame. During each DTC time slot, 324 bits are transmitted, of which the major portion, 260 bits, is due to the speech output of the codec, including bits due to error correction coding of the speech output, and the remaining bits are used for guard times and overhead signalling for purposes such as synchronization.
It can be seen that the TDMA cellular system operates in a buffer-and-burst, or discontinuous-transmission, mode: each mobile station transmits (and receives) only during its assigned time slots. At full rate, for example, a mobile station might transmit during slot 1, receive during slot 2, idle during slot 3, transmit during slot 4, receive during slot 5, and idle during slot 6, and then repeat the cycle during succeeding TDMA frames. Therefore, the mobile station, which may be battery-powered, can be switched off, or sleep, to save power during the time slots when it is neither transmitting nor receiving. In the IS-54B system in which the mobile does not transmit and receive simultaneously, a mobile can sleep for periods of at most about 27 msec (four slots) for a half-rate DTC and about 7 msec (one slot) for a full-rate DTC.
In addition to voice or traffic channels, cellular radiocommunication systems also provide paging/access, or control, channels for carrying call-setup messages between base stations and mobile stations. According to IS-54B, for example, there are twenty-one dedicated analog control channels (ACCs), which have predetermined fixed frequencies located near 800 MHz. Two frequency bands, each about 25-MHz wide (the A- and B-bands), are provided for transmission and reception. The AVCs and DTCs are also located within the A- and B-bands. Since these ACCs are always found at the same frequencies, they can be readily located and monitored by the mobile stations.
It will be understood that a communication system that uses ACCs has a number of deficiencies. For example, the format of the forward analog control channel specified in IS-54B is largely inflexible and not conducive to the objectives of modem cellular telephony, including the extension of mobile station battery life. In particular, the time interval between transmission of certain broadcast messages is fixed and the order in which messages are handled is also rigid. Also, mobile stations are required to re-read messages that may not have changed, wasting battery power. These deficiencies can be remedied by providing a DCC, one example of which is described in U.S. patent application Ser. No. 07/956,640 entitled "Digital Control Channel", which was filed on Oct. 5, 1992, and which is incorporated in this application by reference. Using such DCCs, each IS54B radio channel can carry DTCs only, DCCs only, or a mixture of both DTCs and DCCs. Within the IS-54B framework, each radio carrier frequency can have up to three full-rate DTCs/DCCs, or six half-rate DTCs/DCCs, or any combination in-between, for example, one full-rate and four half-rate DTCs/DCCs. As described in this application, a DCC in accordance with Applicant's invention provides a further increase in functionality.
In general, however, the transmission rate of the DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC slots may not be uniform and may not coincide with the length of the DTC slots. The DCC may be defined on an IS-54B radio channel and may consist, for example, of every n-th slot in the stream of consecutive TDMA slots. In this case, the length of each DCC slot may or may not be equal to 6.67 msec, which is the length of a DTC slot according to IS-54B. Alternatively (and without limitation on other possible alternatives), these DCC slots may be defined in other ways known to one skilled in the art.
Also, U.S. patent application Ser. No. 07/956,640 also shows how a DCC may be defined alongside the DTCs specified in IS-54B. For example, a half-rate DCC could occupy one slot and a full-rate DCC could occupy two slots out of the six slots in each TDMA frame. For additional DCC capacity, additional half-rate or full-rate DCCs could replace DTCs. In general, the transmission rate of a DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC time slots need not be uniform and need not coincide with the length of the DTC time slots.
FIG. 1 shows a general example of a forward DCC configured as a succession of time slots 1, 2, . . . , N, . . . belonging to a particular DCC. These DCC slots may be defined on a radio channel such as that specified by IS-54B, and may consist, for example, of every n-th slot in a series of N consecutive slots. Each DCC slot has a duration that may or may not be 6.67 msec, which is the length of a DTC slot according to the IS-54B standard. The DCC slots shown in FIG. 1 are organized into superframes (SF), and each superframe includes a number of logical channels that carry different kinds of information. One or more DCC slots may be allocated to each logical channel in the superframe.
FIG. 1 also shows an exemplary downlink superframe, which includes at least three logical channels: a broadcast control channel (BCCH) including six successive slots for overhead messages; a paging channel (PCH) including one slot for paging messages; and an access response channel (ARCH) including one slot for channel assignment and other messages. The remaining time slots in the exemplary superframe of FIG. 1 may be dedicated to other logical channels, such as additional paging channels PCH or other channels. Since the number of mobile stations is usually much greater than the number of slots in the superframe, each paging slot is used for paging several mobile stations that share some unique characteristic, e.g., the last digit of the MIN.
For purposes of efficient sleep mode operation and fast cell selection, the BCCH may be divided into a number of sub-channels. U.S. patent application Ser. No. 07/956,640 discloses a BCCH structure that allows the mobile station to read a minimum amount of information when it is switched on (when it locks onto a DCC) before being able to access the system (place or receive a call). After being switched on, an idle mobile station needs to regularly monitor only its assigned PCH slots (usually one in each superframe); the mobile can sleep during other slots. The ratio of the mobile's time spent reading paging messages and its time spent asleep is controllable and represents a tradeoff between call-set-up delay and power consumption.
As such hybrid analog/digital systems mature, the number of analog users should diminish and the number of digital users should increase until all of the analog voice and control channels are replaced by digital traffic and control channels. When that occurs, the current dual-mode mobile terminals can be replaced by less expensive digital-only mobile units, which would be unable to scan the ACCs currently provided in the IS-54B system. One conventional radiocommunication system used in Europe, known as GSM, is already an all-digital system, in which 200-KHz-wide radio channels are located near 900 MHz. Each GSM radio channel has a gross data rate of 270 kilobits per second and is divided into eight full-rate traffic channels (each traffic time slot carrying 116 encrypted bits).
Digital control and traffic channels are also desirable for other reasons described in U.S. patent application Ser. No. 08/147,254, entitled "A Method for Communicating in a Wireless Communication System", which was filed on Nov. 1, 1993, and which is incorporated in this application by reference. For example, they support longer sleep periods for the mobile units, which results in longer battery life. Although IS-54B provides for digital traffic channels, more flexibility is desirable in using digital control channels having expanded functionality to optimize system capacity and to support hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term "macrocell" generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms "microcell" and "picocell" generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments. Each of these types of cells has a base station which transmits at least one control channel. Thus, a number of neighboring control channels are present for a mobile or remote unit to evaluate as a possible replacement for the current serving control channel to which it is locked.
Accordingly, both ACCs and DCCs will be periodically evaluated by the mobile station for possible control channel reselection. Conventionally, for example, when in an idle state (i.e., switched on but not making or receiving a call), a mobile station in an IS-54B system tunes to and then regularly monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a call through the corresponding base station. When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning and subsequent re-tuning to control channels are both accomplished automatically by scanning all the available control channels at their known frequencies to find the "best" control channel. The terms "scan" or "scanning" as used in this document, can refer to, for example, signal strength measurement, actual signal decoding, or any other method of evaluating a signal.
When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this way, mobile stations stay "in touch" with the system. The analog (non-slotted) control channels specified in IS-54B require the mobile stations to remain continuously (or at least 50% of the time) "awake" in the idle state, at least to the extent that they must keep their receivers switched on. Thus, these conventional systems typically evaluate candidate control channels for reselection purposes at some predetermined, fixed intervals.
According to a more recent innovation in cell reselection disclosed in U.S. Pat. No. 5,353,332 to Raith and Muller, each control channel in each cell is configured to broadcast information about the presence, if any, of other cells and the characteristics of those cells including minimum quality criteria, power requirements, etc. Typically, information about the presence of other cells is broadcast about neighboring cells. For instance, a neighboring cell may be adjacent to, overlapping, or non-contiguous from the broadcasting cell. A mobile periodically scans during idle mode the neighboring control channels in the coverage area that the mobile is located in to determine which cell it should be locked to. Each control channel includes a neighbor list. The neighbor list identifies other control channels which mobiles locked to that control channel should periodically evaluate. Thus, a mobile may continuously select cells to be locked to based on the existing location of the mobile and quality criteria (e.g., received signal strength) associated with the cells. The cell to which the mobile may be locked is the cell in which the mobile satisfies the quality criteria associated with the cell.
While in the idle state, and in addition to evaluating control channels as potential reselection candidates, a mobile station must monitor the control channel for paging messages addressed to it. For example, when an ordinary telephone (land-line) subscriber calls a mobile subscriber, the call is directed from the public switched telephone network (PSTN) to a mobile switching center (MSC) that analyzes the dialed number. If the dialed number is validated, the MSC requests some or all of a number of radio base stations to page the called mobile station by transmitting over their respective control channels paging messages that contain the mobile identification number (MIN) of the called mobile station. Each idle mobile station receiving a paging message compares the received MIN with its own stored MIN. The mobile station with the matching stored MIN transmits a page response over the particular control channel to the base station, which forwards the page response to the MSC.
Upon receiving the page response, the MSC selects an AVC or a DTC available to the base station that received the page response, switches on a corresponding radio transceiver in that base station, and causes that base station to send a message via the control channel to the called mobile station that instructs the called mobile station to tune to the selected voice or traffic channel. A through-connection for the call is established once the mobile station has tuned to the selected AVC or DTC.
As noted above, one of the goals of a digital cellular system is to increase the user's "talk time", i.e., the battery life of the mobile station. To this end, U.S. patent application Ser. No. 07/956,640 discloses a digital forward control channel (base station to mobile station) that can carry the types of messages specified for current analog forward control channels (FOCCs), but in a format which allows an idle mobile station to read overhead messages when locking onto the FOCC and thereafter only when the information has changed; the mobile sleeps at all other times. In such a system, some types of messages are broadcast by the base stations more frequently than other types, and mobile stations need not read every message broadcast.
While the innovative system described in the aforementioned patent application provides many benefits in terms of efficient sleep mode operation with respect to paging, additional features may be advantageous. For example, it would be advantageous to provide the capability to temporarily reassign a mobile or remote station to a different paging class to provide the flexibility necessary for changing system conditions in a cell specific manner.